An airplane travels at an unknown elevation before it descends 3 miles to an elevation of 15 miles. find the elevation of the plane before its descen.

18 miles

Step-by-step explanation:

Let's call it's initial elevation x. Because it descends 3 miles, we now have x - 3. The question says it is now 15 miles above sea level, or x - 3 = 15, and adding 3 to both sides shows that x = 18 miles.

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